Pulse oximetry is a common, non-invasive method used in clinical environments to determine arterial oxygen (de-) saturation. Introduced in 1983 to permit accurate and fast assessment of oxygen delivery, it is recognized worldwide as the standard of care inanaesthesiology and is widely used in intensive care, operating rooms, emergency, patient transport, general wards, birth and delivery, neonatal care, sleep laboratories, home care, veterinary medicine and aerospace. Even more, pulse oximetry provides information not only for the blood oxygen saturation (SpO2), but also for heart rate and local vascular irrigation. In commercial devices, either LEDs or LASERs generate the light to be injected into the skin. The backscattered light is then collected by a photo detector (e.g. aphotodiode). These two elements can be placed either side by side on the surface of the tissue, or on each sides of the tissue leading to two pulse oximetry techniques: reflectance and transmittance. Most SpO2sensors use the fingertip or more rarely, the toe, as the measurement site. The reason is that at these locations, the vascular bed is dense. Besides, the body is not too thick at the finger or the toe; transmission photoplethysmography (PPG) is possible, which results in better accuracy than reflectance PPG. The earlobe is also sometimes used, although problems of perfusion variations have been reported. Finally, reflectance PPG is used mostly on the forehead, because of the reflectance of the skull and the relative stability with respect to motion artifacts. However, the pulsation signal is about ten times weaker. Besides, accuracy problems have been reported.
Commercially available SpO2 sensors products are incompatible with comfortable and non-obtrusive long-term monitoring because they are either inconvenient and cumbersome to wear while performing activities like running, cycling or other outdoor activities (for example at the fingertip) or their accuracy and reliability are limited (as for example for the earlobe and the forehead).
The information conveying part in pulse oximetry is the so called ratio of ratios (ROS), which is the ratio of AC and DC components of a red signal divided by the ratio of AC and DC components of an infrared signal. From the signal processing point of view, the most crucial task leading to an accurate SpO2 estimation is therefore the accurate assessment of AC and DC components of the photoplethysmographic signals. Conventionally, this is achieved either in the time domain by extrema location or template matching or in the frequency domain by extraction of the magnitude of specific spectral components [1]. Time domain methods, even in their most advanced implementation, currently based on weighted moving average technique, give a precision of no better than 2%. In contrast, frequency domain methods based on fast Fourier or cosine transform were identified as potentially superior, as described in reference 1: Webster J G, Design of Pulse Oximeters, Medical Science Series, IOP Publishing (1997). Moreover, in highly noisy environments it has been shown in numerous studies of applied signal processing that robust extraction of efficient and salient features of multidimensional times series is often related to an adequate attenuation of harmful noise contributions in a dual domain, such as, for example, the frequency domain or the domain spanned by the principal or independent component of the observed signals (see reference 2: Virag N, Sutton R, Vetter R, Markowitz T, Erickson M (2007), Prediction of vasovagal syncope from heart rate and blood pressure trend and variability: Experience in 1,155 patients. Heart Rhythm, vol. 4, No. 11, pp. 1377-1382).
The use of ECG signal, or more generally the heart beat information, brings along another advantage in processing noisy PPG signal due to low perfusion. Indeed, in order to improve the noise robustness of pulse oximetry under low perfusion, methods have been proposed which process PPG signals in the time domain in synchronization with ECG (see reference 1).
Known methods for monitoring SpO2 based on frequency domain, such as FFT or DCT, typically require a high computational load. Moreover, the signal is analyzed over a window that is constant such that the analyzed signal can be more or less reliable depending on the possible artifacts and the intrinsic heart rate variability, resulting in a less reliable SpO2 estimated value.